In recent years, a high-definition display device has been required. For example, full high-definition (the number of pixels is 1920×1080) has been in the mainstream of home-use television devices (also referred to as a television or a television receiver); however, 4K (the number of pixels is 3840×2160) and 8K (the number of pixels is 7680×4320) will also be spread with the development of high-definition television devices.
High-definition display panels of portable information terminals, such as mobile phones, smartphones, and tablets, have also been developed.
Examples of the display device include, typically, a liquid crystal display device, a light-emitting device including a light-emitting element such as an organic electroluminescent (EL) element or a light-emitting diode (LED), and an electronic paper performing display by an electrophoretic method or the like.
For example, in a basic structure of an organic EL element, a layer containing a light-emitting organic compound is provided between a pair of electrodes. By voltage application to this element, the light-emitting organic compound can emit light. A display device including such an organic EL element needs no backlight which is necessary for liquid crystal display devices and the like, and a thin, lightweight, high contrast, and low power consumption display device can be thus obtained. Patent Document 1, for example, discloses an example of a display device using organic EL elements.